A double diaphragm pump of this kind is described in German laid-open patent application 33 10 131. In this double diaphragm pump the actuating member consists of an axially displaceable actuating rod that projects from the control spool housing and is arranged axially in the control spool. This actuating rod acts in both directions on the control spool, which is retained in its end position by means of spring-loaded retaining balls until the force of the springs arranged coaxially on the actuating rod exceeds the retaining force. The control spool then shoots under the spring force into the opposite control position and effects the reversal of the diaphragm movement. In this way the control spool is moved back and forth between two stable end positions.
Since the movement of the control spool is controlled mechanically by the diaphragms, which are rigidly connected together by a coupling rod, and a snap device moves the control spool back and forth between its two end positions using potential energy, this gives rise to the disadvantage that at very low pump power the control spool tends to stick in an intermediate position and at very high pump power fluttering of the spring mechanism makes precise valve control impossible. Moreover a large number of movable parts is required which slide over one another and therefore need suitable lubrication. The spring on the actuating rod is heavily loaded and must as a rule be made of special steel. Even so it has only a limited life, which results in relatively high repair costs. In addition the cost of assembly is relatively high.
To overcome these disadvantages German laid-open specification 33 10 131 proposes replacing the actuating rod which acts directly on the control spool via the spring by a pilot valve which, controlled by the movement of the diaphragm, acts on the control spool, which is in the form of a piston, with pressure medium in alternate directions, so that only small forces are needed to actuate the pilot valve while the control spool itself is displaced by the pressure medium.
This design has the disadvantage that a large number of sealing surfaces are needed, with corresponding friction and leakage losses, and that here too there is the danger of the valve assuming a non-functioning middle position which can bring the pump to a standstill. In addition a certain minimum pressure of the pressure medium is needed to reverse the control spool, so that, particularly in the case of small double diaphragm pumps, it is not possible to operate at pressures less than 2 bar. With this design it is necessary to make a compromise between low losses of pressure medium, with associated sluggishness, and smooth running with the associated losses of pressure medium. In addition this double diaphragm pump makes heavy demands on manufacturing accuracy, is expensive to assemble on account of the large number of individual parts, and has to consist predominantly of metal.